Final answer:
Compiling a Linux kernel that is different from the existing version will not replace the current kernel and requires managing module compatibility to maintain system stability. Modules must be recompiled or updated to match the new kernel version to avoid functionality issues. Selecting the new kernel version at boot time allows you to use it alongside the old one.
Step-by-step explanation:
Building a Linux kernel that is not the same version as the one currently running on your system is a common practice, especially for developers or users who need specific features or improvements that are present in the newer or older versions. When you do this, essentially you are compiling a new kernel, and this process does not immediately replace the existing kernel on your system. The new kernel will have its own version number, and you can choose to boot into it by selecting it in the boot manager (e.g., GRUB).
However, if the new kernel version has major differences, some modules or hardware drivers may not work as intended since they have to be compatible with the kernel's Application Binary Interface (ABI). It is important to ensure that all necessary modules are also built for the new kernel version. This process can be addressed by recompiling or updating the modules to match the new kernel. Failure to do so may result in system instability or loss of functionality for certain hardware components.
If you try to load a module that was built for a different kernel version, the system will typically reject it due to version mismatch. The kernel keeps track of module versions for this reason. As a result, when upgrading or changing kernels, always make sure to also update or rebuild the associated modules.